The present invention pertains generally to thermoplastic polymer blends comprising a monovinylidene aromatic copolymer in combination with an acetal polymer and with one or more thermoplastic polycarbonate or polyester homopolymer or copolymer resin ingredients. In certain preferred embodiments hereof, the indicated polymer blends also contain as a fourth polymer component therein an elastomeric polymer selected from the group consisting of thermoplastic polyurethanes, copolyester elastomers and combinations thereof. In another preferred embodiment, the monovinylidene aromatic copolymer is a rubber-modified monovinylidene aromatic copolymer having from about 1 to about 40 weight percent of dispersed rubber particles contained therein.
Blends of various and varying types of polymeric materials have been suggested over the years in a variety of prior art references. Of these, the prior art references utilizing acetal resins as blend components generally illustrate compositions in which the acetal resin forms the major or continuous phase, presumably because of large shrinkage and associated interfacial stress build-up occurring upon cooling from the melt. For example, U.S. Pat. No. 4,665,126 to Kusumgar et al. discloses certain polymeric molding compositions containing a predominant amount (e.g., from about 60 to 95 weight percent) of an acetal polymer ingredient in combination with relatively lesser amounts (e.g., from about 4 to 30 and from about 1 to 10 weight percent, respectively) of a thermoplastic polyurethane (TPU) and a multiphase composite interpolymer such as, for example, a butadiene-based, rubber-modified styrene/methyl- methacrylate polymer. Such Kusumgar et al. formulations are said to have improved impact strength relative to that of the acetal polymer per se and relative to that of comparable two component acetal/TPU or acetal/multiphase composite interpolymer blends and to be useful in various molding applications.
U.S. Pat. No. 4,694,042 to McKee et al. pertains to thermoplastic molding polymer blends containing a minor proportion (i.e., from 5 to 50 parts) by volume of a partially or completely crystalline polymer such as nylon, polyacetal, etc. wherein said crystalline polymer, even though employed in minor volumetric proportion, is nevertheless considered to form a coherent phase and wherein the second, major proportion component forms a dispersed phase therein. Within the indicated McKee et al. blends, said major proportion (i.e., from 50 to 95 parts by volume) component consists of one or more crosslinked, emulsion-polymerized elastomeric polymers such as, for example, butadiene or acrylate rubber-based graft copolymers containing either from 10 to 50 weight percent of a shell having a glass transition temperature of less than -10.degree. C. or a substantially lesser amount of a hard polymer shell of styrene, methylmethacrylate or styrene acrylonitrile copolymer. Acetal resin-based compositions are not evident in the working examples.
British Patent 1,311,305 discloses thermoplastic molding compositions composed of a mixture of from 50 to 99 weight percent of an acetal polymer and from 1 to about 50 weight percent of a butadiene or acrylate rubber-modified, two-phase polymer mixture. Such thermoplastic molding compositions are described as having considerably improved impact strength relative to that of the acetal polymer per se. Preferred embodiments of this reference utilize 80 to 95 weight percent of the acetal polymer component.
U.S. Pat. No. 4,639,488 to Schuette et al. discloses impact resistant polyacetal-based molding materials containing from 30 to 95 weight percent of an acetal polymer and from 5 to 70 weight percent of an emulsion polymerized elastomeric graft copolymer composed, on a graft copolymer weight basis, of from 60 to 90 weight percent of a butadiene-based core (or "grafting base") and from 10 to 40 weight percent of a grafted shell of a styrene and/or methylmethacrylate-based polymer or copolymer. Such molding materials are said to have high impact strength at low temperatures, to exhibit good thermal stability and to resist discoloration in the presence of light.
U.S. Pat. No. 4,179,479 to Carter discloses thermoplastic polymer blend compositions containing from 40 to 100 weight percent of a thermoplastic polyurethane in combination with up to 60 weight percent of a thermoplastic polymer which can be an ABS resin, an acetal resin, a polycarbonate resin, a polyester resin or mixtures thereof. Such compositions are also required to contain 0.5 to 10 weight percent of an acrylic polymer processing aid to improve the processability and molding characteristics thereof
U.S. Pat. No. 4,117,033 to Gale discloses polymer blends containing an acetal resin in combination with from 0.1 to 5 weight percent of a low molecular weight copolyether-ester resin. Said copolyether-ester resin is said to improve the melt processability of the indicated acetal resin.
U.S. Pat. No. 4,683,267 to Lindner et al. discloses molding compounds consisting of a mixture of from 60 to 99.00 parts by weight of an acetal resin, from 0 to 40 parts by weight of an elastomer softening below the melting point of said acetal resin and from 0.01 to 40 parts by weight of an aliphatic, rubber-like, high molecular weight adipate-carbonate mixed ester. Elastomers said to be useful in the Lindner et al. blends include homopolymers and copolymers of alpha-olefins, homopolymers and copolymers of 1,3-dienes, copolymers and homopolymers of vinyl esters and copolymers and homopolymers of acrylate and methacrylate esters.
Another publication concerned with blends of polyacetal resins and polystyrene resins is Japanese Kokai No. 64-38463, published Feb. 8, 1989. Such publication is essentially concerned with polyacetal/polystyrene blends wherein the polyacetal constitutes the major portion by weight thereof and requires in all events that the ratio of the polyacetal melt flow rate (MFR, ASTM D-11238 at 190.degree. C. and 2160 g) to the polystyrene melt flow rate (ASTM D-16238 at 200.degree. C. and 5000 g) be from 5:1 to 100:1. According to such publication, excellent surface appearance is obtained by operating within, and only by operating within, the indicated range of polyacetal: polystyrene melt flow rate ratios. Also according to such publication, the polymer blends thereof optionally may contain small amounts of additional polymer ingredients such as a polyurethane resin, an olefinic homopolymer or copolymer resin, acrylate resins, polyamide resin, ABS resins or polyester resins.
Certain blends of rubber-modified styrenic copolymers such as ABS resins with polycarbonate resins are discussed and described in U.S. Pat. Nos. 4,526,926 and 4,624,986 (Weber et al.) and in U.S. Pat. Nos. 4,163,762 and 4,243,764 (Rudd). Not contemplated, however, by said patents are acetal resin-containing (or acetal resin and thermoplastic polyurethane or elastomeric copolyester-containing) blends as are provided in accordance with the present invention.
Published German application DE 3,802,753 A1 discloses polymeric molding materials containing homo- or copolyoxymethylene (POM), thermoplastic polyurethane elastomers (TPU), obtained by the reaction of aromatic di-isocyanates with linear polyols, and polyalkylene terephthalate resin. Such mixtures are described as being useful for applications in vehicles and electrical appliances and to provide improved impact resistance Preferred compositions are from 40-95 percent POM and from 5-60 percent of a mixture consisting of 60-98 percent TPU and 2-40 percent polyalkylene terephthalate.
There remains a continuing need to provide engineering thermoplastic materials having a balance of processability, good aesthetics with no pearlescence and having alternative, advantageous property profiles such as mechanical strength, impact resistance, environmental stress crack resistance, creep and chemical resistance and practical toughness as provided by the present invention.